This disclosure relates generally to thin transparent membranes, and more particularly to a silicon oxygen fluoride (SiOF) membrane that can be used as a pellicle for radiation wavelengths of approximately 157 nanometers, and a process for fabricating such an SiOF membrane.
Optical lithography is evolving to the use of light having a wavelength of 157 nanometers (nm), which enables the patterning of line widths that are on the order of 70 nm. To prevent particulate contamination of photomasks used in such lithography, thin membranes (xe2x80x9cpelliclesxe2x80x9d) could be placed a few millimeters away from the mask in a sealed package, as is done in longer wavelength lithographic systems. A high quality pellicle for 157 nm lithography must satisfy a number of critical requirements, such as high transmission at 157 nm, damage resistance to 157 nm irradiation, and thinness. The thinner the pellicle is, the better it is for keeping the light transmission high and keeping the pellicle from being a substantial factor in the system optics. The pellicle materials currently used at 248 and 193 nm are fluorocarbon polymers, but their 157-nm properties are not satisfactory. Other materials are proposed for use as pellicles at 157 nm. A first example is new polymer materials based on fluorocarbons, that may differ in particulars from the polymers used in 193- or 248-nm lithographic systems. The challenges that must be overcome with these new polymer materials are insufficient transmissivity and photoinduced degradation from use with the 157 nm wavelength. A second example is the use of a modified, fluorinated fused silica, a material which has proven itself highly transmissive at 157 nm. However, practical methods for preparing this material in micron thicknesses have not been found. The use of polishing techniques limits the thinness to about 1 millimeter, causing the finished membrane to become a significant factor in the system optics.
What is needed is a new pellicle that is thin and highly transmissive at 157 nm that does not degrade after use in a 157 nm lithographic system and can sustain normal handling for such devices.
In a first aspect of the present invention, a process is for fabricating a pellicle membrane structure. The process includes depositing an etch mask layer on a backside of a semiconductor wafer, depositing a first layer of pellicle membrane protection material that includes carbon on a frontside of the semiconductor wafer, depositing a layer of membrane material on the first layer of pellicle membrane protection material, and depositing a second layer of pellicle membrane protection material that includes carbon on the layer of membrane material. The process further includes patterning the etch mask layer to form an opening within at least one area of the semiconductor wafer, etching the semiconductor wafer to remove semiconductor material in the opening up to an exposed portion of the first layer of pellicle membrane protection material. The process further includes exposing a structure comprising the patterned etch mask layer, the etched semiconductori wafer, the first layer of pellicle membrane protection material, the layer of membrane material, and the second layer of pellicle membrane protection material to an oxygen plasma until the carbon in the second layer of pellicle membrane protection material and the exposed portion of the first layer of pellicle membrane protection material is removed from the structure.
In a second aspect of the present invention, a pellicle membrane structure includes a layer of crystalline semiconductor material having a backside, a frontside and a periphery with an opening through the crystalline semiconductor material from the backside to the frontside; a layer of pellicle membrane protection material on a portion of the frontside of the crystalline semiconductor material, between the opening and the periphery; and a layer of membrane material having a thickness less than 6 microns that covers the opening and is also on the layer of pellicle membrane protection material.